1. Field of the Invention
This invention relates to an electric source circuit for a telephone set having a control circuit for accessing a dial transmission circuit, effecting key input reception from a dial key pad, setting of the state of a talking line, etc., and more particularly an electric source circuit provided with a battery acting as a back-up power source for memory means for storing control data of the control circuit.
2. Description of the Prior Art
Various types of multifunction electric button telephone apparatus utilizing microprocessors have recently been developed. In these telephone sets, obtaining an electric source for driving the microprocessor, presents a problem. Since an appropriate electric source capable of stably supplying power to such electronic button telephone apparatus is not yet available, a commercial power source has been exclusively used. However, where a commercial power source is used, it is necessary to use connecting lines for connecting the telephone sets to the commercial power source. Moreover, circuit means for stepping down the voltage of the commercial source to the driving voltage of the microprocessor and for rectifying the commercial AC voltage requires a large amount of space which prevents miniaturization of the entire apparatus and increases the cost of manufacturing.
For this reason, a telephone apparatus has been proposed in which office line current is used and a battery is provided acting as a back-up source for the memory means or the like of the apparatus. In such a telephone apparatus, however, there is a problem of how to minimize the degradation or consumption of the back-up battery so as to lengthen its effective life.
Among the reasons that cause shortening of the effective life of the back-up battery are the leakage of the battery while speech is not made (idle time) and consumption of the battery caused by the drop of the office line voltage. The leakage of the back-up battery during idle time is mainly caused by the current flowing through a Zener diode constituting a voltage limiting circuit which is provided for the purpose of stabilizing the source voltage. More particularly, the Zener diode is connected to a line where the output of the back-up battery and the output of the office line voltage flow so that the output battery of the back-up voltage is constantly applied across the Zener diode. Although the operating voltage of the Zener diode is generally set to be slightly higher than the output voltage of the back-up battery, a small current flows through the Zener diode even in a range lower than the operating or yielding voltage of the Zener diode. Thus, this small current is one of the causes that rapidly consume the back-up battery.
Usually the office line voltage is derived out through a plurality of diodes and transistors. However, a diode has a forward voltage drop of about 0.7 V and a transistor has a voltage drop of about 0.01 V-0.3 V. Moreover, the office line voltage often decreases to about 3 V due to the connection state of other telephone sets. In such a case, it becomes impossible to drive a microprocessor or the like with the office line voltage. For this reason, provision of the back-up battery is essential.